This invention is directed to an improved locally compliant flexible circuit film for use as the electrical interconnection system in a high density, backplane connector of the type taught in U.S. Pat. Nos. 4,969,824 and 5,171,154, incorporated herein by reference, and assigned to the assignee hereof. A modified connector for which this improved flexible film may be employed is described in co-pending application, Ser. No. (08/299,222), which application is incorporated by reference. In each case, the prior art generally describes a pair of one-piece connector housing members, arranged for mounting to a first planar electronic device, such as a mother board, where such housing members are spaced apart to define a slot for receiving a second planar electronic device, such as a daughter board, to be electrically interconnected to said mother board by way of a flexible film circuit. In a commercial embodiment of the invention, the respective housing faces, adjacent the planar electronic devices, include channels for receiving a coiled spring, and a flexible film, having electrical circuitry or traces thereon, wrapped about the housing members and coiled springs. The coiled springs provide a normal force to the film to ensure electrical contact with the respective planar electronic devices.
Experience has shown that connectors of this type may exhibit some difficulties with longevity in terms of the number of insertions by the daughter board, as well as reliability of making good electrical connections. One of such difficulties is associated with the nature of the flexible film, and the copper circuitry thereon, the subject of this invention. Briefly, due to the electrical requirements and a need to control the characteristic impedance, for the flexible film a minimum of two metallic layers are required, separated by a dielectric of correct thickness to produce the desired effect by the structure. As evidenced with the teachings of the above mentioned patents, the intent was to use the desirable properties of the canted coil spring, which offers a shallow force deflection curve, as the means of producing the normal force between the planar electronic device and the flexible film connector. However, due to the combination of copper and dielectric layers thicknesses, local compliance between two adjacent pads in the direction normal to the flexed surface, may be insufficient to compensate for even relatively small differences in height or otherwise induced variation of the combination of a printed circuit and the flex coplanarity. The practice has shown that even a small particle, such as a couple of microns in size, between a pair of pads or traces will affect the adjacent pads or traces adversely. This is particularly true in high density connectors where centerline spacings are on the order of 0.05 mils. The local stiffness has been demonstrated such, that a deflection of the pad adjacent to the offending one required a local force of 400 grams to deflect the flex pad 0.0002". Since the canted coil spring applies a pressure to the back of the flex circuit at points incrementally spaced according to the spring winding pitch, and frequently not coincident with the pad center, neither the force available nor the force application points are optimum to deflect a pad which is out of contact due to the surface imperfection in its vicinity. Recognition of these problems, and the solutions offered by this invention, pin pointed two areas of concern, namely, more uniform distribution of force over the area of the circuit pads or traces, and a means for making adjacent pads or traces more compliant to adjust for local coplanarity mismatch.
While the co-pending application addresses the design and construction of the connector by the use of a two-piece, floating housing that ensures a predetermined tension on the flexible circuit film. However, such a unique housing construction does not answer all the concerns with the prior art, particularly as to force distribution and local compliance.
The present invention relates to the latter concerns by the use of a steel backed elastomer, between the coil springs and flexible film, to produce a more uniform force distribution, and to the imposition or creation of slots or slits at least partially through the flexible film, typically a composite of at least two metallic layers with a plastic intermatable layer, in order to allow adjacent pads to move either independently or compliantly to each other, by virtue of local displacement induced by the underlying elastomer. These and other features of this invention will become apparent to those skilled in the art from a reading of the following specification, particularly in view of the accompanying drawings.